Add client secret and session ID identifier types.

[Frinksy]

Resolves: #517

I'm not sure if I've gone the right way about adding the types. I used the opaque_identifier! macro since the new types were similar to the KeyName type. Would it be better to define them manually and/or add constructors to them and validate them simultaneously?

[jplatte]

If there is validation to be done, it should work like ServerName, not KeyName. Maybe we should have a similar macro for that, or adjust opaque_identifier! to support this case.

[Frinksy]

If the changes I made are more in line with how it should be, then I think it would be a good idea to use a macro for it. I haven't ever written macros before, but I would be happy to try. Would it be just adding a macro like opaque_identifier with an extra argument being the validator function?

[jplatte]

Left some comments, but nothing fundamental. Please go ahead with creating the macro 🙂

Would it be just adding a macro like opaque_identifier with an extra argument being the validator function?

Exactly 👍🏼

[jplatte]

Needs a rebase (should be relatively easy).

[jplatte]

I've updated the base branch since this is a breaking change (for ruma-identity-service-api).